Both ac and dc conductivities of nanocrystalline ZnO ceramics with grain size of similar to40 nm were measured as a function of oxygen partial pressure and temperature, and compared with coarsened microcrystalline samples (grain size of similar to5 mum). Nanocrystalline ceramics showed intrinsic bulk properties which turned into extrinsic behavior on coarsening. This is attributed to extreme impurity segregation into grain boundary core in nanocrystalline ceramics owing to the markedly enhanced interface-to-volume ratio. Nonetheless the blocking effect (Schottky barriers) of the grain boundaries in the nanocrystalline sample was negligible compared to that in the coarsened material due to the extreme dilution of the interfacial traps. (C) 2003 The Electrochemical Society.